A spokesman for Volkswagen declined to comment on the reports, saying that as a matter of principle the company would not respond to what he called "rumours and speculation".

In an email, Andreas Lampersbach said: "Volkswagen is working with all its strength to conduct a thorough and merciless investigation of this matter."

VW would hold those responsible for rigging the emissions test to account and discuss technical solutions for the problem with authorities, he said, citing a statement by the company's supervisory board.

"Afterwards we will provide a timetable to modify the vehicles of affected customers. This will take several weeks."